Getting new products to the market faster than one's competition is recognized as a key to gaining a large market share. One area of product development having a significant impact on overall market timing is the making of product and package prototypes for market testing. Such testing usually requires multiple look-like, feel-like, and function-like prototypes for consumers to examine or use.
Package components generally involve plastic parts made in very expensive, multiple cavity, steel molds. For example, most bottles are blow-molded and most bottle closures are injection molded. It usually takes large production quantities to justify the cost of a production mold with many cavities. For smaller markets, or for making only a few hundred test parts, single cavity molds or prototype molds are created. Prototype molds provide important learning on whether the part can be made consistently, as well as to provide a tool that can be used to make test parts.
One method of rapidly prototyping containers or parts is investment casting using patterns generated by rapid prototyping systems instead of traditional injection molded wax patterns. An example of such a pattern is a QUICKCAST.TM. pattern, a Trademark of 3D Systems, Inc. of Valencia, Calif. A hollow plastic pattern is coated with a thin ceramic shell usually by a dipping process. The plastic is burned out of the ceramic shell leaving minimal amounts of ash residue behind. Molten metal is then poured into the ceramic shell to cast a metal part or metal mold for a plastic part. Because the shell has only a small hole for admitting molten metal, it is difficult to inspect the critical surfaces for ash residue. Any ash residue on a critical surface will potentially ruin the metal casting. The molten metal cools and shrinks such that critical surfaces are not reproduced accurately. The larger the parts, the greater the inaccuracy.
An improved method of constructing a fully dense mold is disclosed in U.S. Pat. No. 5,507,336 issued to Tobin, April, 1996. The method comprises placing a pattern within a tube which has a melting point greater than that of the infiltration material which will be used in making the metal mold. A ceramic member is cast between the pattern surfaces and the open end of the tube to transfer the critical pattern surfaces to the ceramic member. The ceramic surfaces are inverse to the pattern surfaces. The pattern is burned out and the ceramic surfaces remains in the tube. The ceramic is then covered with metal powder and an infiltration material from the other end of the tube, and the tube is placed in a furnace to form the metal part over the ceramic surfaces. The metal part has surfaces inverse to the ceramic surfaces. A metal mold results when the ceramic piece is removed. The metal mold has the same shape as the pattern, and is useful for molding plastic parts having an inverse shape. This is an ideal process for parts having exterior critical surfaces.
Tobin's process destroys the pattern from which the ceramic mold is created. A process for quickly forming a ceramic mold pattern which does not destroy the pattern, but which is accurate, is needed. Also, it is often necessary to provide a mating metal mold for plastic part molding. In order to do this, the metal mold may require a shape which is the inverse of the pattern. Thus, the ceramic mold needs to have the same shape as the pattern, and therefore requires an intermediate mold be produced between the ceramic mold and the pattern. As with Tobin's earlier process, any ceramic mold should not be contaminated on its surface so that the resulting metal mold is accurate.
In order to avoid destroying the pattern, it is desirable to use an intermediate mold made of a material which can be discarded or reused as needed to transfer the critical pattern surfaces to the ceramic mold. Wax and silicone rubbers have been used for these purposes. Wax (which is heat reversible) has the disadvantage of being brittle and when removed from the pattern can cause small pieces to break off especially where undercuts and thin features are involved. It also can expand and crack the ceramic when heated. Silicone rubbers need to be cured, and when the ceramic releases heat as it "sets", the silicone rubber can distort and cause inaccuracies to develop in the ceramic pattern. Also, silicone rubber has to be removed from the pattern by air injection or other means which forces the silicone from the ceramic. This can cause the ceramic mold to break especially where undercuts and thin features are involved.
It is therefore an object of this invention to provide a process for making a ceramic mold having the same shape as a pattern, which produces accurate reproductions of a pattern of any size, within a tolerance of .+-.0.005 inches and which does not leave an ash or other residue on the ceramic mold.
It is also an object of this invention to provide a process which uses an elastic, heat reversible material to make an inverse intermediate mold of a pattern and which is not distorted during the forming of a ceramic mold therefrom, but which can be removed easily from the ceramic mold without destroying the delicate features of the ceramic mold.
These and other objects will be evident from the description herein.